Electronic structure of heme model complexes: Looking back at a 35-year research career at Toho University

To elucidate the reaction mechanisms of heme proteins, we examined the physicochemical properties of a wide variety of iron porphyrin complexes by means of ultraviolet-visible, infrared, proton nuclear magnetic resonance, carbon-13 nuclear magnetic resonance, electron paramagnetic resonance, and Mossbauer spectroscopy, as well as X-ray crystallography and superconducting quantum interference device magnetometry. We found that some complexes exhibit previously unobserved properties both in naturally occurring heme proteins and synthetic model heme complexes. These properties were as follows: (1) the electronic ground state of low-spin complexes changes from the common (d _xz, d _yz) ³(d _xy) ² to the less common (d _xz, d _yz) ⁴(d _xy) ¹ if the porphyrin ring is ruffled, (2) the porphyrin ring of such low-spin complexes shows a strong radical character, (3) the deformed porphyrin ring greatly stabilizes in the intermediate-spin state, which is usually quite unstable, (4) 2 types of electronic ground state exist in the intermediate-spin state, and (5) some saddled complexes exhibit novel spin crossover phenomena involving the intermediate-spin state. Some of these properties have crucial roles in the enzymatic process of heme proteins.